How to access all selected files from a file selector in Python?
You can access files for any operator using the file-selector, by defining a files collection of OperatorFileListElement.

Here is a complete example of an operator that opens a file selector and prints the paths you select.

Note: you may want to test os.path.isfile(filepath) since its possible to select directories too.

import bpy

from bpy_extras.io_utils import ExportHelper
from bpy.props import (
        StringProperty,
        CollectionProperty,
        )
from bpy.types import (
        Operator,
        OperatorFileListElement,
        )


class SlideshowAddSlide(bpy.types.Operator, ExportHelper):
    bl_idname = "slideshow.add_slide"
    bl_label = "Add Slide Show"
    files = CollectionProperty(
            name="File Path",
            type=OperatorFileListElement,
            )
    directory = StringProperty(
            subtype='DIR_PATH',
            )

    filename_ext = ""

    def execute(self, context):
        import os
        directory = self.directory
        for file_elem in self.files:
            filepath = os.path.join(directory, file_elem.name)
            print(filepath)
        return {'FINISHED'}

def register():
    bpy.utils.register_class(SlideshowAddSlide)


def unregister():
    bpy.utils.unregister_class(SlideshowAddSlide)


if __name__ == "__main__":
    register()

    # test call
    bpy.ops.slideshow.add_slide('INVOKE_DEFAULT')